1. Field of the Invention
The present invention relates to a system and method for testing multiple radio frequency (RF) signal receivers, and in particular, to testing multiple RF signal receivers with a shared RF signal generator.
2. Related Art
Manufacturing test of receive-only RF signal systems, e.g., global position satellite (GPS) signal receivers, is often implemented by using a single RF signal source to be shared among multiple devices under test (DUTs). Generally, this is easily done, as the signal source typically need only provide a continuous signal, plus this has the advantage of reduced costs.
Referring to FIG. 1, such a test implementation uses a common RF signal source 12 which receives one or more control signals 11 in accordance with which an output signal 13 is provided. This signal 13 is provided to a 1:N, e.g., 1:4, power splitter 14 which divides this signal 13 (in signal power) among its multiple output signals 15a, 15b, 15c, 15d, each of which serves as the test signal for a respective DUT 16a, 16b, 16c, 16d. As will be readily appreciated by one of ordinary skill in the art, such power splitters 14 are well known in the art and can be implemented with virtually any number of output signal ports, or alternatively, by cascading multiple power splitters having fewer signal ports such that the output port of an upstream power splitter provides the signal for the input port of a downstream power splitter, in accordance with well-known techniques. Accordingly, virtually any number of DUTs 16 can be tested using a single RF signal source 12.
However, particularly as an increasing number of DUTs 16 is to be tested, this type of test implementation has a number of problems relating to mutual power differences among the different signals 15 feeding the DUTs 16. In other words, ensuring that each DUT 16 receive its signal with the same power as each other DUT is not trivial. For example, the power splitter 14 will not provide each of the signals 15 at the same power level due to differences among the power divisions being accomplished within the power splitter 14 to its output signal ports 14a, 14b, 14c, 14d. Additionally, manufacturing tolerances and different lengths of the cables providing the test signals 15 will introduce further power differences among the signals 15. Hence, in order to ensure equal signal levels at the DUTs 16, adjustable signal attenuators (not shown) would need to be used and controlled externally. This adds to system costs and complicates the testing due to the external control required, as well as the calibration of each signal path (e.g., cable plus attenuator). Further, if each attenuator is not perfectly matched, i.e., in terms of the characteristic signal impedance, signal reflections can cause further differences among the power levels of the signals 15.
In the case of testing GPS signal receivers, the signal source 12 would normally be a single channel GPS signal generator. Often, it is only desired to test the signal-to-noise ratio (SNR) of each DUT 16, thereby effectively testing the noise figure of the receiver. (If a multi-channel GPS signal generator is used, with multiple signal carriers provided, the carrier-to-noise ratio (CNR) would be tested.) If the noise figure is sufficiently low and the DUT 16 can see a satellite signal at a given SNR (or CNR), it is the digital signal processing that ensures signal locking with the satellites. Since the digital signal processing is tested by the supplier of the signal processing chip (and should, therefore, be assumed to be operating properly), this is often sufficient.
On the other hand, if it is desired to test GPS location lock, an antenna located on the roof of the testing facility can be used to receive and convey actual GPS signals to the DUTs 16. However, as the GPS signals received in this manner will vary, e.g., with weather conditions, this cannot be considered a reliable signal source for testing noise figure, since the input signal level for a given satellite signal is not well defined. Accordingly, this is generally useful only as a test of the ability to determine geographic location, i.e., obtain satellite lock.
Alternatively, multi-channel GPS signal generators (e.g., four channels or more) can be used provide an accurate signal to which the DUTs 16 can lock. Conventional multi-channel GPS signal generators, however, are more expensive than single-channel generators, thereby limiting their use to more expensive manufacturing requirements where repeatable locking to GPS signals must be tested.